An electric power steering apparatus which provides a steering mechanism of a vehicle with an assist torque by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque T of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a voltage control command value Vref obtained by performing compensation or the like with respect to the current command value. The steering angle sensor 14 is not essential and it does not need to be provided.
A controller area network (CAN) 50 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed V from the CAN 50. Further, it is also possible to connect a non-CAN 51 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN to the control unit 30.
The control unit 30 mainly comprises a CPU (also including an MCU, an MPU or the like), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or sent from the CAN 50) are inputted into a current command value calculating section 31 for calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque T being inputted and the vehicle speed V being inputted and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm of which a maximum current is limited is inputted into a subtracting section 32B, and a deviation I (Irefm-Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a PI control section 35 for characteristic improvement of steering operations. A voltage control command value Vref of which the characteristic is improved by the PI control section 35 is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter circuit 37 serving as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38 and fed back to the subtracting section 32B. The inverter circuit 37 uses FETs as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added in the adding section 32A, and characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic or the like. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further, adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
In such the electric power steering apparatus, friction made by the reduction gears and the rack-and-pinion mechanism is large, and an equivalent inertia moment around the steering shaft is large because of the motor for generating the assist torque. Therefore, a steering wheel return worsens because the friction is large in a low vehicle speed range where a self-aligning torque (SAT) is small. This causes a load of a driver, because a steering angle does not return to a neutral position in a going straight state by means of the only SAT, so that it is necessary to return the steering angle to the neutral position by steering intervention of the driver.
On the other hand, the SAT is large in a high vehicle speed range, so that a steering angle velocity tends to increase in the high vehicle speed range compared with the low vehicle speed range. However, because an inertia moment is large in the high vehicle speed range, an inertia torque is also large, so that the steering wheel does not converge at the neutral position of the steering angle, and overshoots the neutral position. This causes the driver to feel a vehicle characteristic unstable.
Accordingly, it is necessary to increase the convergence to assist the steering wheel return in the low vehicle speed range and make the vehicle characteristic stable in the high vehicle speed range. Various control methods for moderately assisting at the time of the steering wheel return have been proposed to achieve them. In the control methods of the steering wheel return, there is an electric power steering apparatus shown in the publication of Japanese Patent No. 4685557 B2 (Patent Document 1) as a prior art for controlling the steering wheel return smoothly even if the driver intervenes in steering.
The apparatus shown in Patent Document 1 calculates a target steering angle velocity by correcting a base-target steering angle velocity with multiplication and addition using a vehicle speed and a torque in a controller that is configured so as to follow the target steering angle velocity. When the driver intervenes in steering, the apparatus decreases an uncomfortable feeling in driver's steering by correcting the target steering angle velocity in the direction where the torque is added.